Cyclic and early gestational uterine differentiation of the normal human and rhesus monkey is being analyzed through electron microscopy in association with cytochemistry and radioautography. Emphasis is being placed on stromal differentiation and the identification of the stem cells for the various uterine cell types in the rhesus monkey. Current effort in normal human endometrium centers on stromal differentiation from 12-18 days of pregnancy. We are analyzing the cytological organization of the human decidual cell during early pregnancy, as well as the lymphoid infiltration that accompanies decidual differentiation. Nuclear differentiation in the uterine cells of the immature and adult rat uterus is being analyzed quantitatively and qualitatively in relation to the number and kind of "nuclear bodies". Our previous findings indicate that "nuclear bodies" appear in the luminal epithelial cells of the immature rat uterus in increasing numbers in relation to the degree of estrogenization. Current effort is directed toward analysis of the antagonistic effect of nafoxidine toward estradiol-induced uterine growth. We also are analyzing uterine nuclear differentiation in adult rat uteri during periods of natural maximal estrogenic stimulation (e.g., day 9 of pseudopregnancy-deciduomata).